The present invention relates to an improved method for preparing nuclear fuel pellets and recycling uranium dioxide and, more particularly, to the processing and recycling of uranium dioxide by utilizing microwave radiation in microwave induction furnaces.
Uranium dioxide (UO.sub.2) is the fuel most commonly used in present day nuclear power reactors with the UO.sub.2 generally taking the form of sintered pellets which are loaded into and sealed in slender, hollow metal tubes called fuel rods. It is a plurality of such fuel rods that establishes an accumulation of fissionable material in sufficient concentration to support sustained fission reactions within the core of a nuclear reactor.
A number of techniques have been developed for manufacturing nuclear fuel pellets the most common of which is the cold pressing of powdered UO.sub.2 into pellets which are sintered in hydrogen in a refractory-type furnace at temperatures of about 1700.degree. C. for approximately three to six hours. Typically, after pressing, the formed pellets are placed in highly heat-resistant transport containers called transport boats which are pushed or walked through a resistance-heated sintering furnace lined with highly refractory blocks wherein the pellets are sintered to form dense, stable end products. Such processing, however, necessitates a special electrically-heated furnace which is expensive to operate and maintain and the temperature range of such "pusher" and "walking-beam" type furnaces seems to be at or about the limit of commercial technology. Additionally, such furnaces have not shown themselves to be amenable to the type of remote operation required for the reprocessing and refabrication of nuclear fuels.
During the nuclear fuel pellet preparation process, there is generated a quantity of rejected sintered UO.sub.2 pellets and scrap UO.sub.2 powder available for recycling. A typical prior art method of UO.sub.2 recovery for recycling is the heating of the material in air so that UO.sub.2 converts to U.sub.3 O.sub.8 and as U.sub.3 O.sub.8 the material is available for recycling or blending back with clean UO.sub.2 and organic binder powders at the initial phase of the nuclear fuel pellet preparation process. U.sub.3 O.sub.8 has a different crystal structure than UO.sub.2 and when it is formed from UO.sub.2 the structural difference causes the initial structure to spall and fragment. Prior art conversion of UO.sub.2 to U.sub.3 O.sub.8 for recycling is commonly done in a conventional air furnace and, as the material is heated, it is shaken to aid in fracturing.
The objects of the present invention are to replace the refractory-type resistance-heated sintering furnace used in the nuclear fuel pellet preparation process and the shaker-air furnace combination used in the UO.sub.2 to U.sub.3 O.sub.8 conversion-recycling process with microwave induction furnaces. Heretofore, microwave induction has been used as a heating mechanism almost entirely via the susceptance of the water molecule to microwave radiation, that is, the use of microwaves for heating of materials has been centered on the effects that microwaves have on water molecules. Microwaves cause rapid changes in the polarization of the water molecule and thereby generate heat. The invention herein described discloses that uranium oxide with stoichiometries UO.sub.2 through U.sub.3 O.sub.8 and that sintered UO.sub.2 pellets and scrap UO.sub.2 powder directly suscept to microwave radiation. Accordingly, the refractory-type sintering furnace and the air furnace above discussed can be replaced by much smaller microwave induction furnaces.
The present invention overcomes many of the fuel processing shortcomings of the prior art refractory-type sintering furnace by decreasing sintering times, allowing quicker attainment of temperatures, producing higher available temperatures, decreasing energy consumption and space requirements, reducing wear on or eliminating the need for refractory furnace materials, readily adapting to modularized operation in remote locations wherein loss of or damage to one module would not shut down the entire operation and resulting in improved densification and porosity characteristics of the sintered product. Additionally, the invention overcomes many of the recycling deficiencies associated with the shaker-air furnace combination by shortening the heat-up time for raising UO.sub.2 to its oxidation temperature, allowing static fragmentation of sintered UO.sub.2 pellets and more efficiently producing a U.sub.3 O.sub.8 powder for blend-back.